1. Field of the Invention
This invention relates to the detection of radar jamming, and particularly to terrain bounce jamming detection using ground clutter tracking.
2. Description of the Related Art
Radar systems typically operate by transmitting electromagnetic energy, or signals, at selected polarization angles. Depending upon the number and size of objects located within the radar environment, some of the signals will bounce off the objects and will be reflected back to the radar receiver. These signals define the object being detected, and thus comprise the "radar skin return," which is reflected off of the skin, or surface, or the detected object.
To mask the radar skin return, and thereby deter or avoid radar detection, certain electronic countermeasures have been developed. Active electronic or electromagnetic warfare operates by generating energy, in the form of noise, to deny effective operation of an enemy's electromagnetic sensors. This energy may be in the form of false or time-delayed signals which can deceive radio or radar devices and their operators. Noise and deception jamming techniques are common forms of electronic warfare countermeasures which operate on the characteristics of the initial electronic or electromagnetic power levels or frequencies. Noise jamming, for example, is the intentional generation of interfering signals which are intended to block a communication or radar system, or at least to impair its effectiveness. Typically, the jamming beam comprises a more powerful signal than the signal being jammed.
A variety of jammers are used today. One jamming technique is direct signal, or direct path, jamming in which an electromagnetic beam of greater amplitude than the jammed signal is transmitted toward the victim radar at the same frequency band as the signal being jammed. By using carefully selected noise modulation, substantial impairment of the intelligibility of reception of the incoming signal may be attained. To counter direct noise jamming, a radar may use a counter technique, called home-on jam (HOJ), to track the angle of the jammer signal. However, because the application of HOJ will home in on a reflected signal, it has been found that HOJ is ineffective against a terrain bounce jammer.
Terrain bounce jamming, in contrast to direct signal jamming, is used to provide a jamming signal from an angle different from the true jammer angle, and thus is generally more difficult to counter. The noise terrain bounce jammer presents a false target angle to the seeker, and therefore directs the missile interceptor away from the true target. A terrain bounce jammer may be implemented on an aircraft, missile, or other air devices. It is especially effective if the jammer is flying at a low altitude and transmits a noise-like continuous waveform toward the ground.
A conventional missile-borne radar may be implemented for initial detection and tracking of targets and possible jamming scenarios. This is generally a monopulse radar transmitting a pulsed signal which, upon return, indicates the range of the target, and doppler shift, and thus the relative velocity of the target, and the angle of the target. However, it has been found that terrain bounce jamming is often effective against monopulse radar systems since using HOJ, the radar homes in on the jamming signal without knowing precisely whether the signal is from the direct path or the target or a reflection off the ground. Thus, the tracking radar may not be able to differentiate whether the received jamming signals are being transmitted by a direct path jammer or a terrain bounce jammer.
Thus, it can be seen that to counter the effects of terrain bounce jamming and, therefore, detect and intercept the real target, HOJ has been found to be ineffective for counterjamming. Using conventional HOJ, the seeker will determined, albeit incorrectly, that the target is located at some point on the ground from which the jamming signals appear to be reflected. As a result, the seeker will direct itself to hit the ground, rather than the rear target. Consequently, a detection and recognition scheme which can recognize the occurrence of terrain bounce jamming is desired.